Author: David Rasmussen
Date: 12:43:33 01/13/02
Go up one level in this thread
On January 13, 2002 at 13:20:52, Dan Newman wrote:
>On January 13, 2002 at 08:48:13, David Rasmussen wrote:
>
>>Even though I try to keep my program extremely portable and ANSI/ISO C++
>>compliant, except for the parts that can't be, I would like to see how much
>>faster my program would be with an inline assembler version of my FirstBit()
>>function for use in VC++ (jeez, when will 64-bit processors be common...).
>>
>>My board setup is a1=0,...,h8=63. For some reason, I can't get it to work.
>>
>>int FirstBit(BitBoard bitboard)
>>{
>> __asm {
>> bsr edx, dword ptr bitboard
>> mov eax, 0
>> jnz l1
>> bsr edx, dword ptr bitboard+4
>> mov eax, 32
>> jnz l1
>> mov edx, -1
>> l1: add eax, edx
>> sub eax, 1
>> }
>>}
>>
>>This is written from me head, as I have deleted what I previously did, so I
>>haven't even tried to compile this, but you get the idea. The problem was that
>>for some bitboards, it returned the correct value, but for some other bitboards,
>>bsr just returned some weird wrong number, which made the entire function return
>>some weird wrong number. What's the problem?
>>
>
>
>I don't know what BSF/BSR do to the flags, but if bit zero is set to one,
>then BSR will put zero into edx and you won't get the jnz jump. Also,
>when there are no bits set BSR leaves the register undefined (in practice
>it leaves it unaffected), so I suspect the flags may not be set one way or
>the other.
>
>I do something like this (Dann Corbit's trick):
>
> bsf eax dword ptr bitboard+4
> add eax 32
> bsf eax dword ptr bitboard
>
I don't understand that. 32 is added to eax no matter what. So the result is
always at least 32.
Where can I see this code in action?
/David
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